Discovery of Peripheral nerve injuries are not uncommon following surgery, and nowadays constitute a major source of postoperative anesthetic-related malpractice claims and postoperative morbidity. This problem has been the subject of considerable interest in recent years, with a number of review articles being published. The exact cause is unclear, but may involve stretching of nerves or more commonly extrinsic pressure applied over a nerve in such a way that it is compressed, usually against a bone. Thus, for a period of hours, the nerve is subjected to mechanical stresses, and remains in this compromised state throughout surgery. If the duration of surgery is long and the extrinsic force high, the nerve may be irreversibly injured. Lesser degrees of injury may result in immediate loss of function with gradual return of nerve function over weeks or months.
It is not known what combination of extrinsic pressure and duration results in irreversible nerve injury. However, it is commonly understood that leaning on an elbow can rapidly make the little finger go numb (due to mechanical forces operating on the ulnar nerve), and that sitting cross-legged for a while can numb the leg and foot due to pressure on the sciatic nerve. When we are awake, we recognize these warning symptoms and shift the weight off of the nerve, thereby relieving the pressure so that permanent injury does not result. These anecdotal reports suggest that the extrinsic pressure necessary to injure nerves may be regularly found in common situations, and may be due to the patient's own weight. Further, these reports also suggest that the forces necessary are not trivial, i.e., numbness does not normally occur without an identified provocation.
There are a number of sites where nerves may be compressed leading to postoperative neuropathy:
1. Ulnar nerve beneath the medial epicondyle (posterior inner aspect of the elbow). Extrinsic pressure here can compress the ulnar nerve against the medial epicondyle and/or olecranon. PA1 2. Radial nerve over the posterior aspect of the arm midway between the shoulder and the elbow. The radial nerve here can be compressed against the humerus. PA1 3. Peroneal nerve can be compressed as it passes around the fibula several inches below the outside of the knee. PA1 4. The sciatic nerve can be compressed in the buttock as it passes between the ischium and the trochanter. PA1 5. The femoral nerve in the groin can be compressed as it exits beneath the ilioinguinal ligament to enter the thigh. It can be compressed against the pubic ramus. PA1 6. Lateral femoral cutaneous nerve of the thigh can be compressed by pressure against the anterior iliac spine. PA1 1. beneath the elbow if patients are lying on their back, PA1 2. under the midpoint of the arm if additional pressure is applied there, PA1 3. over the sciatic nerve in the buttocks if patients are in the sitting position, PA1 4. over the femoral nerves or lateral femoral cutaneous nerves for patients in the prone position, PA1 5. The sensationless skin of spinal cord injury patients.
Therefore, it is understood that external forces can damage nerves, however, suitable methods and systems for determining when such forces are sufficient to cause damage are not readily available.